1. Field of the Invention
The present invention relates generally to printed circuit boards, and more particularly to a printed circuit board having improved resistance against solder bridging and component decentering/tombstoning.
2. Disclosure Information
A conventional printed circuit board (PCB) generally includes a substrate 10 with conductive circuit traces 30 and mounting pads 14 thereon, as shown in FIGS. 1-3. Once the traces and pads have been disposed on the substrate, a thin, solder-resistant layer of material, often called a "solder mask" 16, is applied to the surface of the PCB. Typically the solder mask covers substantially all of the circuit traces, but not the mounting pads; this is accomplished by forming apertures 18 in the solder mask wherein each aperture generally conforms in shape with and is situated concentrically about a respective mounting pad. For example, if a given mounting pad is rectangular, its corresponding solder mask aperture will likewise be rectangular, with the edges 22 of the aperture evenly spaced apart from the corresponding edges 28 of the mounting pad by a given amount or gap P. This gap P between the mounting pad edges 28 and aperture edges 22 is referred to as "solder mask pullback"; for screenprinting, the pullback P is typically about 10-20 mils, whereas for liquid photoimageable solder mask (LPISM) the pullback P is typically 0-5 mils. As illustrated in FIGS. 1-3, this pullback of the mask from the mounting pad creates a "trough" 32 about each pad.
After the solder mask is applied to the PCB surface, the mounting pads and any other circuit structures exposed through the apertures may be tinned, followed by solder paste being applied thereto, typically by screen-printing or deposition. Solder paste is typically a slurry of metallic solder particles (e.g., eutectic tin/lead), flux, and other components. After the paste has been printed/deposited, and the PCB has been populated by placing electronic components thereon with the component terminations atop their corresponding mounting pads, the populated PCB is subjected to reflow processing.
During reflow, the metallic component of the solder paste melts and forms solder joints, while the flux and other effluent components of the paste are liberated and often flow across the solder mask surface about each solder joint. Various gases and vapors are also ejected from the molten paste during reflow and solidification. It is well known that when flux and other effluents flow across the solder mask, they provide solder-wettable paths along which solder balls may be transported. This presents a serious problem, because these migratory solder balls may end up solidifying in such a way as to short out two or more adjacent mounting pads; or, enough of the solder or paste may be wicked away from a pad such that the solder that is left thereon is insufficient to form an acceptable joint (or any appreciable joint at all), resulting in an open circuit, or a weak joint if one forms at all. Additionally, if flux or other effluents wick up underneath the component 24, the component may "skate" or "float" on the flux/effluents and become unacceptably decentered with respect to the mounting pads. Furthermore, gases escaping from any paste which wicks up under a component may escape so violently as to promote or worsen the tendency for the component to decenter or tombstone.
Therefore, it would be advantageous to provide a way of channeling away and containing the flux and other effluents liberated during reflow, in order to preclude the flux/effluents from promoting or exacerbating the aforementioned problems of solder ball migration and component decentering and tombstoning.